Resonant actuator systems are used in a variety of different applications, such as to move optics within cameras by way of example only. Examples of resonant actuator systems may be found in U.S. Pat. No. 6,940,209, titled, “Ultrasonic Lead Screw Motor”; U.S. Pat. No. 7,339,306, titled, “Mechanism Comprised of Ultrasonic Lead Screw Motor”; U.S. Pat. No. 7,170,214, titled, “Mechanism Comprised of Ultrasonic Lead Screw Motor”; and U.S. Pat. No. 7,309,943, titled, “Mechanism Comprised of Ultrasonic Lead Screw Motor,” which are hereby incorporated herein by reference in their entireties. In these different applications, control over the velocity of operation of the resonant actuator systems often is required.
One method of velocity control of resonant actuator systems is to control the drive voltage. In this method, the drive voltage (typically between about 16 volts and about 40 volts) is varied by a controller, which results in a corresponding variation in the rotational and linear velocity of the threaded element of the motor. Drive voltage is the most power efficient method of regulating the motor speed, but often is louder and less precise than desired for particular applications. Additionally, controlling velocity with drive voltage is not well suited for motion control applications in which two or more motors are required. For example, two motors are needed for dynamic motion control in applications such as optical image stabilization (OIS) of digital cameras. The dynamic motion for OIS applications must correct for hand tremors and is generally a mix of sinusoidal waveforms with amplitudes up to 1 mm and frequencies up to 20 Hz. This dynamic motion control must be achieved without generating significant acoustic noise. Two axes of motion, and thus two independently controllable motors are required. When using drive voltage control with a single controller in communication with two motors, both motors share the same voltage. As a result, independent speed control of each motor by a single controller is not possible using drive voltage control.
Another method of velocity control of resonant actuator systems is “burst mode.” This method uses a fixed drive voltage and outputs periodic “bursts” of motor drive waveforms to the motor. Shorter bursts result in a lower velocity and longer bursts result in a higher velocity. This method provides a wide range of speed control, and may be applied independently to two motors. However, it also creates significant audible noise in the moving motor, as well as a higher rate of wear of the motor components.